1. Field of the Invention
The invention relates to a method for gasifying organic materials and mixtures of materials as defined in the invention.
2. The Prior Art
A process for gasifying organic substances and mixtures of substances is known from U.S. Pat. No. 4,568,362 (1), in which the organic substances are admitted into a pyrolysis reactor, in which these substances come into contact with a heat-carrying medium. Such contact leads to a high rate of pyrolysis and the substances are converted into pyrolysis products, i.e. pyrolysis gases containing condensable substances and solid, carbon-containing substances. The thermal energy required for the pyrolysis stage is generated by combusting the solid, carbon-containing residue. In a second reaction zone, the tar-containing pyrolysis gases are subjected to cracking reactions and reaction with steam in such a way that a gas product with a high calorific value is obtained.
In said process, both the pyrolysis and the combustion of the solid, carbon-containing residue take place in a fluidized bed. In the upper part of the fluidized-bed pyrolysis reactor, provision is made for a reaction zone for the tar-containing pyrolysis gases. The heat-carrying medium is partially discharged together with the solid, carbon-containing residue via the reactor head of the fluidized-bed reactor, and the remaining part is discharged via a conduit arranged at the borderline of the upper fluidized bed, and supplied to the firing stage of the fluidized bed. There, the solid, carbon-containing residue is burnt, and the heat-carrying medium is heated up. The heated-up heat-carrying medium and the ash are jointly discharged with the exhaust gas from the fluidized-bed firing stage, separated in a gas/solids separator located above the fluidized-bed pyrolysis reactor, and supplied to the reaction zone of the pyrolysis reactor, from where they drop again into the fluidized bed of the pyrolysis reactor (=the heat-carrying medium circulation).
The operation of such fluidized beds, however, requires a substantial amount of expenditure, and it is hardly possible to exert any influence on the reactions of the pyrolysis gases occurring in the reaction zone. Furthermore, highly superheated steam has to be used in the reaction zone, which in turn requires the use of water that has been treated at substantial expenditure.
A process for gasifying organic substances and substance mixtures is known from DE-PS 197 55 693 (2). In this process, the organic substances are brought into contact with a heat-carrying medium in a migrating-bed reactor, which leads to rapid pyrolysis, with conversion of the organic substances into a carbon-containing, solid residue, on the one hand, and a pyrolysis gas that consists of condensable volatiles and gaseous components on the other.
The heat-carrying medium and the pyrolysis coke are subsequently supplied to a combustion stage, in which the carbon-containing residue is burnt, on the one hand, and the heat-carrying medium is heated up, on the other hand, before it is recycled into the pyrolysis stage.
After adding a reactant, which is steam, as a rule, the tar-containing pyrolysis gas is after-heated in a second reaction zone realized in the form of an indirect heat exchanger in such a way that a gas product with high calorific value is obtained. The heat exchanger takes place is indirectly heated by means of the combustion gases as the latter are being cooled. Following the firing process, the ash is separated from a partial stream of the mixture consisting of the heat-carrying medium and the ash of the solid, carbon-containing residue, and then cooled and discharged.
Said process, however, has a number of aspects that make a device for carrying out this process complicated in terms of the required expenditure and costly as well, and may also have an adverse influence on the operation and on the availability as well. First of all, the heat-carrying medium in transported in the heated state from the combustion stage back into the pyrolysis stage, i.e. at a temperature that is by far above the pyrolysis temperature, which is specified to amount to from 550° to 650° C. This makes it imperative to employ special conveying means that require a particularly high amount of expenditure in terms of material and mechanically speaking. Furthermore, to the extent to which the heated heat-carrying medium is still mixed with ash, it has to be expected that the latter will escape and thus cause baking problems. Secondly, the indirect heat exchanger, owing of its operating conditions that include temperatures of from 500° to 1000° C. on both sides, requires reducing conditions, on the one hand, and because of the highly corrosive components contained in both in the pyrolysis product and product gas and in the combustion exhaust gas as well, requires materials that require substantial expenditure, as well as an additional purification system that also may require much expenditure because of softening of the ash may possibly occur, on the other hand. The risk of ash baking to surfaces in the heat exchanger sets narrow limits for the operation and design of the firing stage as well. A further problem is encountered when steam is added to the pyrolysis gases: the steam is either superheated, which requires a great amount of expenditure, or the temperature is lowered, which may lead to condensation of tar and consequently to baking problems. Finally, situations are conceivable in which it is not possible to assure a defined heat transfer into the heat-carrying medium as it is heating up again in the firing process, so it has to be feared that the pyrolysis coke and the heat-carrying medium are de-mixed in the firing stage, so that, for example in the case of a grate firing process, the pyrolysis coke is burnt off on the layer on top, whereas the heat-carrying medium may still be cooled by the current of grate air streaming in through the grate from the bottom.